1. Field of the Invention
The present invention relates to a coal pulverizer associated with a rotary classifier applicable to a fuel feed system of a pulverized coal burning boiler or the like.
2. Description of the Prior Art
In a heretofore known coal pulverizer associated with a rotary classifier applicable to a pulverized coal burning boiler, a method for controlling the rotary speed of the rotary classifier either set the speed to a constant value as indicated by a curve a in FIG. 5 or set the speed to a number of values as a function of a coal feed rate as indicated by the segments of curve b.
Under a constant coal feed rate, if the rotary speed of the rotary classifier is increased, the grains of pulverized coal at the outlet of the coal pulverizer become fine. Also, a load on the coal pulverizer increases and a motor current also increases. On the contrary, if the speed of the rotary classifier is decreased, the grains of the pulverized coal at the outlet of the coal pulverizer become coarse. Also, a load on the coal pulverizer and the motor current decrease.
In the event that a property (brand) of the coal fed to the coal pulverizer has changed, for instance, in the case where it has changed from high grindability (soft coal) to low grindability (hard coal), a motor current of the coal pulverizer increases for the same coal feed rate. In the case of very hard coal, sometimes the motor current exceeds a rated current, resulting in a motor trip. On the other hand, in the case where the coal is very soft, it is desirable to obtain an as high as possible degree of pulverization to achieve a high efficiency in the operation of the boiler. To this end, it is necessary to increase a motor current of the coal pulverizer.
In view of these considerations, a method shown in FIG. 6 has been proposed. In this method, the rotary speed of the rotary classifier is manually changed depending upon the brand of coal used.
However, since all of the coal is fed at once to a large hopper known as a "coal bunker", it is difficult to precisely know when the brand of coal fed to the pulverizer has changed. In addition, it is not rare for the property (especially grindability) of coal to vary greatly throughout even coal of the same brand. Therefore, even the method of FIG. 6 cannot properly establish the speed of the rotary classifier.